1. Field of the Invention
The present invention relates to a circuit for processing a video intermediate frequency signal and, more particularly, to a video detection circuit for demodulating the video intermediate frequency signal to generate a video signal.
2. Description of the Background Art
FIG. 1 is a block diagram schematically showing the circuit structure of a conventional television receiver of an intercarrier system. Referring to FIG. 1, a television broadcasting signal received by an antenna 11 is converted to a video intermediate frequency signal having video carrier frequency f.sub.p (45.75 MHz in U.S.A.) by a tuner 13. The video intermediate frequency signal includes a sound intermediate frequency signal component having sound carrier frequency f.sub.S1 (41.25 MHz in U.S.A.). The video intermediate frequency signal is amplified by an intermediate frequency amplifier 15 and thereafter received by a video detection circuit 17 which in turn detects the video intermediate frequency signal to output a video detection output signal including a demodulated video signal component and a frequency-converted sound intermediate frequency signal component having sound carrier frequency f.sub.S2 (4.5 MHz in U.S.A.). The sound intermediate frequency signal component is removed through a sound trap circuit 19, and thus only the demodulated video signal component is processed by a video circuit 21 to be provided to a picture tube 23. On the other hand, only the sound intermediate frequency signal component is extracted through a sound filter 25. A sound frequency demodulation circuit 27 demodualtes the extracted sound intermediate frequency signal to generate a sound signal. This sound signal is amplified by a sound amplifier 29 to be outputted to a speaker 31.
FIG. 2 is a block diagram showing the typical circuit structure of the intermediate frequency amplifier 15 and the video detection circuit 17. The video intermediate frequency signal from the tuner 13 is inputted in a bandpass filter 33 which is formed by a surface acoustic wave filter, for example. The bandpass filter 33 has a bandpass characteristic shown in FIG. 3, wherein a passing amount is -6 dB at frequency f.sub.p and varies along a liner inclination in a frequency range of f.sub.p .+-.0.7 MHz. It is well known that a normal video detection output can be obtained through the bandpass filter 33 having such a bandpass characteristic, as described in the document "Circuit Design of Television Receiver, pp. 125-127, published by RAJIO GIJUTSUSHA in Japan in 1968", for example.
The video intermediate frequency signal filtered by the bandpass filter 33 is amplified by an amplifier 35. The amplifier 35 is controlled by an automatic gain control system so that the output of the amplifier 35 is always maintained at a predetermined appropriate amplitude even if the amplitude of the inputted to television broadcasting signal varies. The output of the amplifier 35 is received by a phase locked loop (hereinafter referred to as PLL) circuit 37 and a synchronous detection circuit 39. The PLL circuit 37 comprises a voltage controlled oscillator (hereinafter referred to as VCO) 41, a phase comparator 43 for phase-comparing outputs of the VCO 41 and the amplifier 35, and a loop filter 45 for filtering the output of the phase comparator 43 to supply the filtered output to the control input of the VCO 41. When the PLL circuit 37 is locked, the output of the VCO 41 has the same frequency and a difference in phase of 90.degree. as compared with the video carrier (frequency f.sub.p) of the video intermediate frequency signal. The output of the VCO 41 is phase shifted by 90.degree. by a phase shifter 47 to become the same in phase with the video carrier of the video intermediate frequency signal, and then received by the synchronous detection circuit 39. On the basis of this received signal, the synchronous detection circuit synchronous-detects the video intermediate frequency signal outputted from the amplifier 35, to derive the video detection output signal.
In the conventional video detection circuit hereinbefore described, the bandpass characteristic of the bandpass filter 33 has the inclination in the vicinity of frequency f.sub.p, as shown in FIG. 8. As a result, the video carrier signal is subjected to phase modulation by the AM component of the video intermediate frequency signal, as described in the above document and Japanese Patent publication Gazette No. 61-11030, for example. The output of the VCO 41 exactly follows the video carrier signal as phase-modulated. The synchronous detection circuit 39 synchronous-detects the video intermediate frequency signal on the basis of this output of the VCO 41. Thus, a phase modulation component is mixed in the sound intermediate frequency signal component included in the video detection output signal. The phase modulation component& becomes a sound buzz when the sound intermediate frequency signal is frequency-demodulated.
To solve this problem, a circuit shown in FIG. 4 is proposed, wherein an additional circuit 40 for a sound signal is provided in parallel with the video detection circuit 17 shown in FIG. 2. The additional circuit 40 has the same circuit structure as that of the video detection circuit 17 with the exception that a bandpass filter 33a has bandpass characteristics shown in FIG. 5. In this additional circuit 40, a video carrier signal of frequency f.sub.p is extracted without being subjected to phase modulation, through the bandpass characteristic A of the bandpass filter 33a which is symmetrical with respect to the frequency f.sub.p, and amplified by an amplifier 35a to be received as a reference signal by a PLL circuit 37a comprising a VCO 41a a phase comparator 43a and a loop filter 45a. The output of the VCO 41a is phase-shifted by 90.degree. through a phase shifter 47a to be received by a synchronous detection circuit 39a. On the other hand, a sound intermediate frequency signal having sound carrier frequency f.sub.S1 is extracted through the bandpass characteristic B of the bandpass filter 33a, which is symmetrical with respect to the sound carrier frequency f.sub.S1, to be received by the synchronous detection circuit 39a after amplified through the amplifier 35a. The synchronous detection circuit 39a converts in frequency the received sound intermediate frequency signal on the basis of the output of the phase shifter 47a to output a sound intermediate frequency signal of sound carrier frequency f.sub.S2 (=f.sub.p -f.sub.S1). The output of the VCO 41a includes no phase modulation component since the output of the bandpass filter 33a includes no phase modulation component as hereinbefore described. Therefore, the sound intermediate frequency signal outputted from the synchronous detection circuit 39a includes no phase modulation component, and hence no sound buzz is caused when this sound intermediate frequency signal is frequency-demodulated.
However, this system requires two circuits of the structure shown in FIG. 2, to complicate a circuit structure and increase a cost. Further, two VCOs 41 and 41a are required, to weaken resistance to noise and increase the number of external parts necessary in integration.